Aromatic polyester resins are used in ever-increasing amounts in the production of beverage containers and films.
The barrier properties of aromatic polyester resins are rather limited. In the case of containers for carbonated beverages prepared from said resins, the possibility to preserve the beverages for a sufficiently long time is uncertain.
Polyamide resins have remarkable mechanical properties, but they have the drawback that they have a high moisture absorption which reduces their properties.
Polyamide resins are normally used mixed with aromatic polyester resins in order to improve the mechanical characteristics of the latter. The presence of polyester resin in the mix reduces the moisture-absorbing tendency of polyamide resins.
Mixing the resins, however, is difficult because of their poor compatibility in the melted state.
In order to obtain better mechanical properties and to avoid peeling in products, it has been suggested to mix the resins in the extruder in the presence of a dianhydride of a tetracarboxylic acid (JP 1-272660 Kokai).
Pyromellitic anhydride is the preferred compound.
The mechanical properties of the resulting mixtures can be improved further by subjecting the mixtures to a solid-state polycondensation treatment (WO 94/09069).
Among polyamides, the most commonly used polyamides, such as nylon 6 and 66, have slightly better gas barrier properties than polyester resins such as polyethylene terephthalate (PET) and copolyethylene terephthalates such as polyethylene terephthalate (PET) and copolyethylene terephthalates containing small proportions of units derived from isophthalic acid.
However, a polyamide obtained from m-xylylene diamine and adipic acid (poly-m-xylylene adipamide, poly MXD-6) is known as having considerable gas barrier properties (in relation to oxygen and carbon dioxide) which are distinctly better than those of polyethylene terephthalate.
This polyamide is used in mixture (obtained in an extruder) with PET or COPET in order to improve their barrier properties.
The oxygen permeability of a 1.5-liter PET bottle (produced by injection blow-molding) is reduced by approximately 50% when the bottle is obtained from a mixture which contains 16% by weight of polyamide and by approximately 20% when it contains 7% polyamide by weight.
The barrier properties of the 16% polyamide mixture are similar to those of a bottle formed of a two-layer film, one layer being PET and the other one being polyamide.